Holographic gun sights are well known, but typical designs are complex and may be bulky or have high energy usage. Adjusting a holographic gun sight for windage and elevation has also presented challenges. Adjustment is required to align the positioning of a reconstructed reticle and to compensate for various weapon types and targeting procedures. Existing systems have drawbacks. Accordingly there exists a need in the art for an alternative or improved design for holographic gun sights.
Laser diodes are used in a wide variety of applications that require a narrow spectral width. However, the wavelength of the light produced by the laser diode varies depending on a number of factors, including change in the temperature of the laser diode. For example, some laser diodes will exhibit a shift in output wavelength of approximately 0.30 nm/° C. The change in the temperature of the laser diode may be due to environmental conditions or due to heating from operation of the diode. For some applications, this shift in wavelength is not a problem. However, for other applications, such as a holographic gun sight, this shift in the output wavelength may cause the holographic gun sight to be inaccurate.
In a holographic gun sight, the hologram reconstructs an image of a reticle which will appear in focus at a distance in the viewing field (Virtual Image Plane). The sight is typically designed so that this image will overlap the target. Holographic diffractive optics may be wavelength dependent and the thus may be very sensitive to a change in the laser diode wavelength. As the output wavelength shifts, the diffraction angle from a holographic element may change, which may result in a movement of the projected holographic image resulting in an inaccurate reticle position relative to the target.
To correct for this change in the output wavelength some sights have achromatic holographic elements which compensate for changes in the output wavelength. However, it remains desirable (simpler design, easier manufacturing, more reliable, lower cost) to provide a source of laser light in which the output wavelength is stable as the temperature changes within an operating range. Similar considerations apply to other devices utilizing laser light, such as a stable LED, RCLED . . . etc.
One approach to addressing this problem is to control the temperature of the laser diode, such as through a use of a thermoelectric device or TEC cooler. Such control may be open or closed loop. An open loop control may be used, such as a temperature sensor attached to the laser diode. As the sensor temperature changes, the TEC cooler keeps the diode at one stable temperature. For a closed loop system, the wavelength output of the laser diode may be directly monitored by a device such as a grating. This information is then used to adjust the temperature of the laser diode via the thermoelectric cooler, and bring the diode back to a desired wavelength. While thermal control of the laser diode is effective in preventing a change in wavelength, thermoelectric controllers are large in comparison to the laser diode and may draw a current in excess of 0.5 amps. For either case, using a thermoelectric cooler increases the physical size of the laser source assembly and greatly increases its energy requirements. For this reason, thermoelectric controllers in gun sights are impractical and undesirable at this time. For example, US Patent Application Publication No. 2014/0064315 discloses another means for incorporating a thermoelectric controller in a gun sight.
An alternative type of semiconductor laser diode is known as a VCSEL, or vertical-cavity surface-emitting laser. A VCSEL has improved temperature stability as compared to a standard laser diode. For example, a VCSEL may have a wavelength shift of approximately 0.05 nm/° C., which is approximately a six-fold improvement over a standard laser diode. While this is a large improvement over the standard laser diode, even this smaller amount of wavelength shift may be enough to impair the accuracy of a device using the VCSEL.
Parallax mismatch is another undesired result of using holography for target shooting. Parallax mismatch results when the reconstructed reticle, also referred to as a perceived image, is displayed on an object either closer or farther than the intended distance. This means that the reconstructed reticle will appear to move around as the viewing eye moves. If the reconstructed reticle is displayed on an object at the predetermined distance from the hologram apparatus, then the image should not move. It is desired that the reconstructed reticle remains still as the viewing eye moves about a display hologram when the reconstructed reticle is displayed on objects at varying distances. This effect would improve shooting accuracy and precision. Accordingly, there is a need for a holographic weapon sight that compensates for parallax mismatch.
A dual H.O.E. assembly can present several challenges. For example, if the H.O.E. alignment in use does not match near perfectly with how the H.O.E. was recorded, then it results in an undesired imaging. Accordingly, a need exists to provide weapon sights with improved holographic imaging in a compact and convenient setting.